Schottky diodes are employed as rectifiers in numerous power and small signal applications where the forward conduction or switching characteristics of the diode are important. Conventional silicon PN-junction diodes have a number of disadvantages, including: (i) a large voltage drop of approximately 700 mV or more, depending on the operating current density when the diode is in its forward conducting state, and (ii) the diode's characteristic of storing a large number of minority carriers when it is in the forward-conducting condition, which slows the diode's turn off time and leads to numerous problems relating to diode reverse recovery such as increased power loss, heating, noise and reduced circuit efficiencies.
Fast recovery times are achieved in high voltage PIN diodes by reducing minority carrier lifetime through irradiation or platinum doping and in low voltage applications (below 100V) using a metal-silicon Schottky barrier diode instead of a PN junction. At low voltages, the Schottky diode is preferable to the PN junction diode because of its lower voltage drop.
A major disadvantage of the Schottky diode is its relatively high offstate leakage current, which is typically orders of magnitude higher than the leakage current of a PN junction diode. Moreover, the leakage current in a Schottky diode is strongly dependent on the applied reverse voltage, as a consequence of reverse-voltage-induced-barrier-lowering at the rectifying metal-semiconductor interface, sometimes referred to as the “Schottky interface” or “Schottky contact”.
The offset leakage current is a strong function of the Schottky barrier height, which can be influenced by the choice of the metal forming the contact to the semiconductor. Metal electrodes with a higher work function, resulting in a higher Schottky barrier height, show a lower leakage current. However, a higher Schottky barrier leads to a larger forward-voltage drop, which varies linearly with the barrier height.
The barrier-lowering which has a dominant effect on the leakage current at high reverse voltages is itself a function of the value of the electric field peak appearing at the Schottky interface. The value of the peak electric field in turn is a strong function of the doping level of the semiconductor, and increases with increasing doping concentration. As a result, conventional planar Schottky diodes are over-designed in the sense of using a doping concentration in the semiconductor that is too low for the requested blocking voltage. In the end, a lower doping of the semiconductor means a larger serial resistance of this region and a larger forward voltage drop of the device.
Thus an important issue in the design of a Schottky diode is the optimization of the trade-off between the voltage drop in the conducting state and the leakage current in a reverse-bias condition. Many of the inventions for improving the performance of this device are focused on a reduction of the electric field at the Schottky interface and introduce device structures which allow avalanche breakdown to occur far away from the Schottky contact.
An early attempt to reduce the trade-off between forward and reverse currents is the junction-barrier-controlled Schottky (JBS) rectifier as described in U.S. Pat. No. 4,641,174 to Baliga et al., shown in cross-section in FIG. 1. The JBS rectifier is a merged Schottky/PN diode having a PN junction grid interspersed between the Schottky contacts. The PN junction grid is designed to deplete the Schottky channel regions under reverse bias conditions, but not to pinch-off these channels to forward conduction current.
The purpose of the PN grid is to change the electric field distribution within a reverse-biased device in a way that the Schottky contacts are shielded against high electric peak values and breakdown occurs at the bottoms of the P-wells. The dimensions of the grid and the doping levels are such that the depletion layers extending from the PN junctions into the substrate merge under the Schottky contacts when the reverse bias exceeds a few volts. Any further increase of the reverse-bias voltage is supported by the depletion layers of the PN junctions, away from the Schottky interface. The efficiency of the shielding of the Schottky interface by the described mechanism depends on the aspect ratio Tc/Wc of the channels. However, the implantation and diffusion of the PN junctions leads to a significant loss of the Schottky contact area and does not allow high aspect ratios (Tc/Wc) because of the lateral diffusion of the P-type dopant. For these reasons, the JBS concept has not been used for commercial products.
An attempt to develop the JBS concept can be found in U.S. Pat. No 4,982,260 to Chang et al. The proposed improvements of the usage of the active area of the device and an attempt to increase the Tc/Wc aspect ratio of the channel are sketched in FIGS. 2 and 3. In the device from FIG. 2 the lateral diffusion of the PN junctions is limited by MOS trench regions. However, it can be clearly seen, that this concept does not improve the usage of the active area of the device in a significant manner. Another embodiment shown in FIG. 3 includes PN junctions formed at the bottom of MOS trench regions. Here, the proposed device structure requires a very complex and expensive technology, which is prohibitive for large volume products.
A simplification of the structure of FIG. 3 is described in U. S. Pat. No. 5,365,102 to Mehrotra and Baliga, and is presented in FIG. 4 in this application. In this device the PN junctions have been replaced by a grid of trenches with a MOS gate. For an appropriate doping and width Wm of the mesa regions, the MOS trenches are efficient enough to shield the Schottky contacts and allow the design of conducting channels having a high aspect ratio. This trench-gated Schottky diode fulfills the design requirements from the point of view of the optimization of the forward voltage drop at reduced leakage currents. However, this concept disregards the problems caused by the generation of “hot” minority carriers at the bottom of the trench in avalanche breakdown conditions. The minority carriers generated during avalanche breakdown can have a kinetic energy high enough to be injected into the gate oxide. An accumulation of injected hot carriers impairs the gate oxide and in time can permanently damage the device. Such devices have no energy absorbing capability for avalanche surge currents.
An avalanche rugged, trench-gated Schottky diode is described in the above-referenced application Ser. No. 08/832,012 to Williams et al. A device designed according to this concept is sketched in FIG. 5. It includes a grid of MOS trenches separating mesa regions with Schottky contacts and integrated clamping PN diodes which have a lower breakdown voltage than the trench structure. An epitaxial layer with a stepped or graded doping profile is preferred. It should be noted that the definition of a specific breakdown voltage of the device and the requirement of a lower doping in the vicinity of the MOS trench may not allow a free optimization of the doping of the channel mesa regions.
What is needed is a Schottky diode structure minimizing the barrier lowering effect, integrating an avalanche rugged PN clamping diode, and allowing an easy fabrication.